1. Field of the Invention
The invention relates to a method as well as to a device for regulating the formation of a fibrous web, in particular a paper, cardboard or tissue web, whereby formation of the fibrous web is measured online as a control variable and is kept to a predefined set point level via automatic formation control.
2. Description of the Related Art
The fiber distribution and composition in a fibrous web is described as formation. The formation is generally also described as the “look-through” for example of the paper.
Inspection and evaluation of the formation generally occurs in that the paper is X-rayed. The structure and the level of uniformity of the fiber distribution in the paper is a measure for the formation. A distinction is hereby made between uniform, placid and clear look-through for paper of good quality and irregular, cloudy and very unsettled look-through for lesser quality papers.
The formation of a paper or cardboard web is a decisive quality characteristic which influences the further processing of the fibrous web in a multifaceted way. One example is the influence of the formation on the strength and the printability of the end product.
The formation of a paper or cardboard web is influenced on the one hand by the raw material used and on the other hand to a not inconsiderable extent by the sheet formation. The sheet formation is hereby determined by the headbox and the forming unit. The forming unit may for example comprise a Fourdrinier or a hybrid former or a modern twin wire former.
The values which influence the formation in this process step may for example be stock consistency, vacuums in the forming unit, the retention and the water volume, as already also described in DE 10 2005 062 304 A1.
Experience shows that even minor changes in the composition and the characteristics of the used fibrous stock can have considerable influence on the formation. Moreover, each change in basis weight of a paper web requires readjustment of the values influencing the formation.
Because of these aforementioned reasons the formation is subject to continuous fluctuations. These formation fluctuations are especially pronounced in products containing waste paper, which can be ascribed to the corresponding fluctuations in the raw material composition.
Today, spontaneously occurring formation changes are countered mainly through mechanical adjustment of the headbox lip opening. This alters the stock consistency in the infeed to the headbox, so that the basis weight can be kept constant.
In order to improve the formation an enlargement of the lip opening is necessary, leading to higher energy use, since the throughput volume is increased. As a result however, the overall ash-retention decreases disadvantageously, requiring the addition of more retention agents.
Also affected are the control circuits for automatic dosing of retention agents, as well as the efficiency of the white water filtering downstream units.
The drop in retention makes the addition of increased volumes of retention agents necessary, thereby also negatively impacting the formation due to stronger flocculation. A process fluctuation is released by these self-exciting control circuits which destabilizes the entire water management system of the paper machine (stock metering station, approach flow system, process water recovery, stock preparation) over several hours.
To automatically control the formation, several online formation measurements and controls are disclosed in the current state of the art, for example in EP 1 342 843 A1, DE 101 18 508 A1 or DE 10 2005 062 304 A1. It is for example suggested to keep the formation to a predefinable setpoint level with the assistance of the dewatering efficiency in the forming unit or by means of the retention agent adjustment.
What is needed in the art is to create an improved method as well as an improved apparatus of the type described at the beginning with which the formation of a fibrous web can be stabilized at a higher level.